In the production of aluminum cans, for example, a strip of aluminum is fed to a die press from a coil in an uncoiling machine. The press punches out shallow, cup-shaped blanks from the strip. The cup-shaped blanks are "drawn" and "ironed" to form the bottom of a two piece aluminum can. The bottom is then mated with a separate top.
The press utilizes a compound die set in a two step operation. It punches out a circular blank and then forms the blank into a cup. The press formed cup is two (2) to three (3) inches high.
The short cup is then drawn out so that it has an elongated tubular body. The open end of the tubular body is ironed outwardly to form a lip. The top of the can is then attached to the lip.
These forming steps can be successfully carried out only if the strip of aluminum has a coating of lubricating fluid on it. Accordingly, it is conventional to apply a coating or film of lubricant to the surface of the aluminum strip before it enters the press operation.
Where relatively low speed die operations are involved lubricant film thickness can vary considerably without untoward effects. For example, older presses, which run at ninety (90) strokes per minute or less, require minimal control of lubricant film thickness. With these older, slow presses the lubricant is merely sprayed or wiped onto the aluminum strip.
With modern presses, where the speed is increased to between one hundred eighty (180) and two hundred forty (240) strokes per minute, however, control of lubricating film thickness becomes critical. Anywhere there is insufficient lubricant the die may blow through the cupping operation and rupture the aluminum strip. Anywhere there is too much lubricant the grippers which hold the cup while it is being drawn may lose their grip whereby the cup is not drawn properly.
To satisfy the needs of the faster, modern presses, various improved methods of applying lubricant were developed to supplant the traditional spraying or wiping operation. For example, a process involving electrostatic deposition of lubricant was developed. Another approach which was developed is called "roll coating" and involves the mechanical application of a lubricant film with the aid of lubricant coating rolls.
In the electrostatic deposition process, control of lubricant film thickness is quite precise. However, with this process film consistency begins to break down where stopping and starting of the strip is necessary, as it frequently is in practice. In such case the lubricant tends to "puddle up", i.e., form small pools on the strip surface and become uneven in thickness across the surface of the strip.
"Roll coating" machines have been relatively successful in applying a coating of lubricant which is uniform enough for the operation of modern presses. They normally include a first set of propulsion rolls which drive a strip of aluminum into a dip tank containing the lubricant. A second set of propulsion rolls pulls the strip out of the tank and "squeegees" off excess coating fluid. The second set of rolls also serves to compress the film or fluid against the strip and form a micro-fine film of lubricant on both the top and bottom of the strip. Each of the second set of rolls is polyurethane coated to a thickness of about one-half inch (1/2") so that they are compressible.
The second set of propulsion rolls drives the lubricated strip out of the roll coating machine into a holding loop between the machine and the press feed rolls. Upon demand from the press, feed rolls drive the strip into the press where the multiple cup forming operations are performed. The press feed rolls serve to further refine the thickness of the lubricant film, compressing the film against the surface of the strip just before it is fed to the die press. The result is a relatively well controlled lubricant film thickness and distribution.
The press feed rolls play an important part in controlling the lubrication film on the strip when a conventional roll coating machine is employed. In order to assure satisfactory lubrication film uniformity, they correspond in length to the width of the strip, i.e. they are full width rolls.
With conventional roll coating machines, full width rolls are also preferred because they flatten out edge burrs, which the roll coating rolls cannot do. Strips three (3) to four (4) feet in width are now conventional and strips as wide as five (5) or six (6) feet can be used with some of the newer presses. As the widths increase the rolls increase in length, accordingly. As long as press speeds do not exceed approximately one hundred and eighty (180) strokes per minute the use of such massive, high inertia feed rolls is not a problem. However, presses are now being constructed which operate at much higher speeds. As press speed increases the mechanical strip feed indexing mechanism can no longer cope with repetitively overcoming the substantial inertia of the feed rolls and the indexing mechanism tends to break down frequently. To prevent this, larger, more expensive cam drives must be employed.
Of course, the mass of the feed rolls can readily be reduced by shortening the rolls. Doing so, however, shortens them to a length less than the width of the strip. Where the feed roll does not contact the strip across its entire width, it has been found impossible to achieve a final uniform thickness of lubricating film utilizing conventional roll coating machines. The effect is to practically limit the use of conventional roll coating machines to press feeds of approximately two hundred and forty (240) strokes per minute or less.
Conventional roll coating machines also have other important shortcomings. The second set of propulsion rolls have a plastic, compressible material on their surface, as previously described. After a certain amount of use the plastic gets flat spots. As a result, the lubrication film thickness varies.
The fact that the rolls are covered with a relatively compressible material creates another problem. Both sets of propulsion rolls are driven through a gear train from the press at a speed directly related to that of the press. The pitch diameter of the gears which drive them determines the rotation speed of the propulsion rolls. As long as the surface diameter of each set of rolls is related identically to the pitch diameter of the drive gears the roll coater operates properly.
However, when the rolls of the second set of propulsion rolls squeeze together to grip the strip between them the roll diameter is changed because of the compressibility of the plastic surface. The relationship between the pitch diameter and the roll diameter vary with the amount of pressure applied. The two sets of propulsion rolls attempt to drive the strip at slightly different speeds. As a result, the second set or lubricant applicator rolls tend to slide on the surface of the strip, creating a wiping action and gaps in the lubricant film.
Still another problem which conventional roll coating machines have been unable to cope with satisfactorily relates to the fact that the aluminum strip has a coating of mill oil on it when it comes from the coil manufacturer. The coating is a lubricant itself so it is not inherently unattractive. Unfortunately, mill oil is not a substance which blends readily with water soluble lubricants employed in coating the strip prior to press forming. The adhesion characteristics of the mill oil are different than those of the water soluble lubricant. As a result, the squeegee action of the second set of propulsion rolls in a roll coating machine acts differently on patches of mill oil than it does on unoiled, dry strip lubricant. Puddling of the mill oil occurs and uneven lubricant film application results.
Other problems are also inherent in the gear drive and compressible surface propulsion roll relationship inherent in conventional roll coating machines. As press feed speed increase with increasing productivity requirements all problems are magnified. Solutions have not been found in traditional approaches.